Search for old Spec, GT, Marin, Orange or Saracen (Saracen made good bikes once) on the bay. I turned up an old Univega for not much on RetroBike for a budget MTB/child carrying bike. Worth checking there as well.
My dad had an awesome Claud butler 531 lugged mtb, with full deore LX... must have been a 91 or 92 model. We didn't think much of it once we had "proper" mountain bikes and gave it away to family, might try and retrieve it.
i'd love to replace my work bike with a oldschool MTB frame, the problem is i know fuck all about them so i dunno what to look for